The invention concerns generally an apparatus for guiding a wire electrode on a machine tool, and in particular is an apparatus for controlling the axial tension of the wire electrode in the working zone.
In conventional spark-erosive cuffing machines, an upper guide head moves horizontally in the axial direction, and a wire electrode inclines by cooperative movement of the upper and the lower guide head. As the upper and the lower guide head move during the process or eroding cutting there may occur (1) wearing-down of the wire electrode, (2) loosening of the wire electrode, (3) complex forces on the wire electrode, (4) insufficient flow of water, (5) partial change in plastic property of the wire electrode, and finally (6) breaking of the wire electrode. The angle for cutting work is limited to a small angle, 15 degrees at the most. In order to solve this problem, the upper and the lower guide head should incline according to the inclination of the workpiece.
U.S. Pat. No. 4,431,896 uses gimbal suspensions for mounting the guide heads and motors for controlling movement about the respective gimbal axes for accomplishing inclination of the guide heads in accordance with that of the workpiece placed in the working zone.
Another known U.S. Pat. No. 4,485,288 makes use of two parallel sets of four connecting rods, which are moved to orient the upper and the lower guide heads in accordance with the angle of a workpiece.
Another known U.S. Pat. No. 4,667,078 has a wire electrode looped around a grooved wheel controlled by a hysteresis motor near an upper or lower guide head while the wire drive is located near the other guide head so that in the working zone, the electrode is subject to the action of tensile stress necessary for electroerosion. The remaining electrode guide area is kept free from increased tensile stress on the electrode. Additionally, an angle coder is used with the hysteresis motor to measure both the rotational speed and the position of the grooved wheel.
Although the three above mentioned devices for guiding a wire electrode have their own advantages, their structures are generally too complicated to have many control elements, and are difficult to maintain and repair.